Tadalafil is a selective inhibitor of cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type 5 (PDE5). Tadalafil has the empirical formula C22H19N3O4 representing a molecular weight of 389.41. The structural formula is:

It is a crystalline solid that is practically insoluble in water and very slightly soluble in ethanol.
CIALIS (Tadalafil) is available as almond-shaped tablets for oral administration. Each tablet contains 2.5, 5, 10, or 20 mg of Tadalafil and the following inactive ingredients: croscarmellose sodium, hydroxypropyl cellulose, hypromellose, iron oxide, lactose monohydrate, magnesium stearate, microcrystalline cellulose, sodium lauryl sulfate, talc, titanium dioxide, and triacetin.
After single oral-dose administration, the maximum observed plasma concentration (Cmax) of Tadalafil is achieved between 30 minutes and 6 hours (median time of 2 hours). Absolute bioavailability of Tadalafil following oral dosing has not been determined. The rate and extent of absorption of Tadalafil are not influenced by food; thus CIALIS may be taken with or without food.
Over a dose range of 2.5 to 20 mg, Tadalafil exposure (AUC) increases proportionally with dose in healthy subjects. Steady-state plasma concentrations are attained within 5 days of once-daily dosing, and exposure is approximately 1.6-fold greater than after a single dose. Tadalafil is eliminated predominantly by hepatic metabolism, mainly by cytochrome P450 3A4 (CYP3A4). The concomitant use of potent CYP3A4 inhibitors such as ritonavir or ketoconazole resulted in significant increases in Tadalafil AUC values.
The mean apparent volume of distribution following oral administration is approximately 63 L, indicating that Tadalafil is distributed into tissues. At therapeutic concentrations, 94% of Tadalafil in plasma is bound to proteins. Less than 0.0005% of the administered dose appeared in the semen of healthy subjects.
Tadalafil is predominantly metabolized by CYP3A4 to a catechol metabolite. The catechol metabolite undergoes extensive methylation and glucuronidation to form the methylcatechol and methylcatecholglucuronide conjugate, respectively. The major circulating metabolite is the methylcatechol glucuronide. Methylcatechol concentrations are less than 10% of glucuronide concentrations. In-vitro data suggests that metabolites are not expected to be pharmacologically active at observed metabolite concentrations.
The mean oral clearance for Tadalafil is 2.5 L/hr and the mean terminal half-life is 17.5 hours in healthy subjects. Tadalafil is excreted predominantly as metabolites, mainly in the feces (approximately 61% of the dose) and to a lesser extent in the urine (approximately 36% of the dose).
The main medical concerns surrounding Tadalafil are related to slow absorption and variable onset of action with tmax values in the 0.5-6 hour range.
A variety of strategies have been used to attempt to overcome these issues, see for example WO 2014125343, WO2006049986, WO2012085927, WO2006091974, WO2007027612, WO2008134557, WO2012095151, WO2014092661, WO2014125352, WO2011135426, WO2016012539, WO2013109221, WO2013109223, WO2016001143, WO2014168455, WO2014202797 WO2016021975, WO2010115886 and WO2014209022.